From: Hugh Dickins <hughd@xxxxxxxxxx> In a kernel with added WARN_ON_ONCE(PageTail) in page_memcg_check(), we observed a warning from page_cgroup_ino() when reading /proc/kpagecgroup. This warning was added to catch fragile reads of a page memcg. Make page_cgroup_ino() get memcg from compound_head(page): that gives it the correct memcg for each subpage of a compound page, so is the right fix. I dithered between the right fix and the safer "fix": it's unlikely but conceivable that some userspace has learnt that /proc/kpagecgroup gives no memcg on tail pages, and compensates for that in some (racy) way: so continuing to give no memcg on tails, without warning, might be safer. But hwpoison_filter_task(), the only other user of page_cgroup_ino(), persuaded me. It looks as if it currently leaves out tail pages of the selected memcg, by mistake: whereas hwpoison_inject() uses compound_head() and expects the tails to be included. So hwpoison testing coverage has probably been restricted by the wrong output from page_cgroup_ino() (if that memcg filter is used at all): in the short term, it might be safer not to enable wider coverage there, but long term we would regret that. Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx> --- (Yosry: Alternatively, we could modify page_memcg_check() to do page_folio() like its sibling page_memcg(), as page_cgroup_ino() is the only remaining caller other than print_page_owner_memcg(); and it already excludes pages that have page->memcg_data = 0) --- mm/memcontrol.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index 5abffe6f8389..e3a55295725e 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -395,7 +395,7 @@ ino_t page_cgroup_ino(struct page *page) unsigned long ino = 0; rcu_read_lock(); - memcg = page_memcg_check(page); + memcg = page_memcg_check(compound_head(page)); while (memcg && !(memcg->css.flags & CSS_ONLINE)) memcg = parent_mem_cgroup(memcg); -- 2.40.0.rc1.284.g88254d51c5-goog
